What have you been doing in the trunk lately? I heard about a guy partying in his trunk. :) If you've been doing a lot of hauling lately you may have caused something to knock a wire loose in the trunk. Check for that. Also look for signs of moisture. You may need to pull the bulbs and dry them out and then pull the whole assembly and replace the moisture seal. Since the right side gets power from the left side light assembly, check that side for a loose wire as well.

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check the light bulbs (filiments can break and touch the other filament in the bulb and backfeed other circuits) if they appear ok remove the non working bulbs and swap positions with the working bulbs and see if anything changes

Check your Tailight fuse to see if it's blown, if so then start checking for a bad bulb(shorted element) in tailight circuit boards . and also don't forget to check grounds as well, there should be one at rear frame by left (drivers Side) tailight connection/trailer harness connection. hope this helps. do your Brake lights Work ?

I was once a fleet manager with many Jeep Grand Cherokees 1999-2002. We had TREMENDOUS problem with REAR tailight housings. It seem the tailight/signal/back-up light circuitry is embedded in the plasticx of the housing. NO WAY to repair. Replaced with a new one from Jeep. Problem gone. ( at least until the other side went bad ) Not a guarranty, but it's a good chance it has the same problem.